This invention relates to alkaline earth metal carbonates and, more particularly, is concerned with a process for producing precipitated alkaline earth metal carbonates.
The precipitated alkaline earth metal carbonate which is manufactured in the largest quantities is precipitated calcium carbonate, optionally associated with varying proportions of magnesium carbonate. Precipitated calcium carbonate and calcium/magnesium carbonate are white pigments used extensively in paper coating compositions and as fillers or extenders for paper, paint, rubber or plastics.
One known method of preparing a precipitated alkaline earth metal carbonate is to calcine a naturally occurring alkaline earth metal carbonate, such as limestone or dolomite, in order to drive off chemically combined carbon dioxide and leave the alkaline earth metal oxide. The alkaline earth metal oxide is slaked in water to form a suspension of the alkaline earth metal hydroxide, and then pure carbon dioxide is passed under controlled conditions through the aqueous suspension of the hydroxide. This process has the disadvantage that, if the raw alkaline earth metal carbonate contains hydrated iron oxides as impurities, the iron is retained throughout the process and is present in the final product, with the result that the product has a yellowish discoloration and a relatively low brightness, or reflectance to visible light. This problem has been recognized for a long time and several attempts have been made to overcome the problem, see for example U.S. Pat. Specification No. 1,994,271 (Church et al) and U.S. Pat. Specification No. 2,520,850 (Minnick). However, the results achieved in practice have not been satisfactory and, for this reason, much naturally occurring limestone and dolomite has been considered unsuitable for use as a raw material for making precipitated alkaline earth metal carbonates.